My son hit his head on the school bus and split it open

Obviously, a blow to the head can be very serious. It makes sense to monitor him to see what if any long term harm has taken place. Be sure to report this to the school if they don't already know. If there are any long terms costs for medical care or any long term injury to your son, you can possibly sue for damages (depending on where and how he was sitting and where he hit is head. both the bus manufacturer and the school are potential defendants